In recent years, positive resist compositions have been mainly used as resist compositions for forming semiconductor devices. The reason for it is that a negative resist composition generally has high sensitivity, but involves a drawback in resolution because it is developed with an organic solvent, so that it swells to a significant extent. On the other hand, positive resist compositions are believed that they can be satisfactorily accommodated to the integration of semiconductors to a high degree because they are generally excellent in resolution. The positive resist compositions heretofore commonly used in this field are those composed of an alkali-soluble phenol resin such as a novolak resin, and a quinonediazide sulfonic acid type photosensitive agent. These positive resist compositions can be developed with an aqueous solution of an alkali and so undergo no swelling and have excellent resolution. In recent years, the resolution of such a positive resist composition has been more enhanced by improvement in its own performance and the development of a higher-performance aligner, so that formation of a minute pattern having a line width of 1 .mu.m or smaller has come to be feasible.
With respect to various properties such as sensitivity, film loss after development, resolution, thermal-flow resistance and storage stability, however, satisfactory results are not necessarily obtained from the conventional positive resist compositions. It is thus desired that the performance of the positive resist compositions be further enhanced. In the formation of, in particular, a minute pattern having a line width of 0.8 .mu.m or smaller, preferably 0.5 .mu.m or smaller, it is necessary to more strictly control resist dimensions. There is accordingly a strong demand for development of a positive resist composition capable of achieving higher dimensional accuracy. There is also a demand for development of a positive resist composition which undergoes little dimensional changes by variations of exposure and focal deviation of exposure light, namely, is good in the so-called exposure margin or focus margin.
From such a point of view, it has been proposed to use, as an additive for modifying resist properties, various kinds of phenolic compounds for positive resist compositions comprising an alkali-soluble phenol resin and a quinonediazide sulfonate type photosensitive agent (Japanese Patent Application Laid-Open Nos. 200252/1991, 122938/1992, 127374/1993 and 232697/1993). However, the positive resist compositions specifically disclosed in these documents are somewhat insufficient in resist properties such as sensitivity, resolution and film loss after development. There is thus a demand for development of a further improved positive resist composition. The improvement of these various properties is extremely important in processing of, in particular, gates, holes, wiring layers and the like, which are difficult to control dimensions because of high reflectance of substrates.
In order to prevent the reduction of accuracy in dimensional control by reflection of exposure light from a substrate, there has also been proposed a methods of reducing the reflectance of a substrate by forming a film on the substrate with an inorganic substance by a CVD process or a PVP process or by coating the substrate with a film of a resin to which a suitable light absorbing component has been added. However, it has not been yet successful to achieve sufficient resist properties on such a substrate by the conventional positive resist compositions. There is therefore a demand for development of a positive resist composition capable of achieving satisfactory resist properties even on such a reflectance-reduced substrate.